The invention relates to a battery with a stack of bipolar individual battery cells of the type defined in greater detail in the preamble of claim 1.
Batteries represented by a stack of bipolar individual battery cells are generally known. The cells are stacked on top of one another and typically clamped together, so that one terminal post comes to lie on one side of the stack of bipolar individual battery cells, while the other terminal post lies on the other side of the stack of bipolar individual battery cells. Such individual battery cells are, for example, described in the German application number DE 10 2007 063 181.4. The individual battery cells are designed as so-called frame flat cells with two sheet metal covers, which are electrically isolated from each other by an insulating frame. The sheet metal covers serve as the terminals of the individual battery cell and as heat-conducting plates that divert the waste heat generated in the battery to the outside of the individual battery cell. There, the sheet metal covers may, for example, be connected to a cooling apparatus that is typically cooled actively by a cooling fluid or by the refrigerant of an air conditioning system.
In the interior of the individual battery cell, i.e., between the sheet metal covers and surrounded by the electrically insulating frame, the electrochemically active materials are located. These typically consist of a stack of electrodes. This electrode stack comprises anode and cathode foils, separated by an electrically insulating separator placed in between, which are alternately stacked on top of one another. At the edges of the metallic foils, the anode foils project on one side, while the cathode foils project on the other side. These regions, which are designated as current-carrying lugs in the above application, are then suitably connected to one another. This may, for example, be achieved by welding as described in the above application. As connecting regions of the electrode stack, these regions are further electrically connected to the respective sheet metal cover. This, too, can be achieved by welding according to the above application. The individual battery cell is then completed by way of the frame and the two sheet metal covers and assembled. In this process, the individual battery cell is sealed against the environment. The interior of the frame is then filled with a suitable electrolyte which, together with the electrode stack, forms the chemically active part of the individual battery cell.
In lithium ion batteries, the anode or cathode foils respectively are typically represented by aluminum or copper foils, and a liquid electrolyte containing lithium ions is typically used as an electrolyte.
In order to connect the respective electrode foils to the respective sheet metal cover, a suitable welding method is used. This welding of the electrode foils or of their connecting regions to the respective sheet metal covers is disadvantageous because material is typically thrown up or raised in the region of the welds, which then prevents a flat contact between one individual battery cell and the sheet metal cover of the adjacent individual battery cell. Corresponding surface irregularities in the material will also be found on the side of the electrode stack. However, this is irrelevant here, because this structure is distanced from the opposite sheet metal cover by the electrolyte-filled region of the individual battery cell.
In order to obtain a flat contact of the sheet metal covers of adjacent individual battery cells even in the weld region, some reworking is required, which makes production complicated and expensive. If the projecting regions of the welds were simply left on the sheet metal covers, the result would be an uncontrolled air gap at this point as well as a point contact between the sheet metal covers and the thrown-up material of the welds. At the relatively high voltages in the stack of individual battery cells, this may even result in sparking in this region. This has to be avoided at all costs if the structure is not to be damaged accordingly.
Exemplary embodiments of the present invention are directed to a battery with a stack of individual battery cells, which avoids the disadvantages described above and which allows for a structure which can be implemented simply and cost-effectively.
According to the invention, the sheet metal cover welded to the electrode stack and/or the opposite sheet metal cover of the adjacent individual battery cell is/are set back in the region of the weld. This creates a distance between the two adjacent sheet metal covers at least in the region of the weld. In the region of the weld, these can then no longer contact one another, so that a reworking of the welds can be omitted without having to risk the above disadvantages. In principle, it is possible to set back only the sheet metal cover with the weld in this region and to leave the opposite sheet metal cover straight, or vice versa. It would also be conceivable to set back both sheet metal covers in a suitable manner.
In this context, it is irrelevant whether both of the sheet metal covers are welded to the electrode stack, which will typically be the case, or whether only one of the sheet metal covers is welded to the electrode stack and the other sheet metal cover is electrically connected to the electrode stack by other means. A variant according to the invention can be used even if there is only one weld.
In a particularly useful further development of the invention, the sheet metal covers and the electrode stack are joined by a pressure welding process. In this method, for example in ultrasonic welding or resistance pressure welding, the electrode geometry is to some extent impressed into the region of the welded components. In ultrasonic welding, for example, this may be a ribbed or knurled surface of the sonotrode and/or the anvil. These pressure welding methods, on the other hand, offer the advantage that different materials can be welded to one another. As, for example in the construction of the individual battery cell with a lithium ion cell chemistry, sheet metal covers made, for example, of iron-based materials have to be welded to aluminum foil anodes or copper foil cathodes, this advantage of pressure welding is critical in the production of such individual battery cells. The production of individual battery cells can therefore be simplified, which makes the production of the battery itself more cost-effective.
Advantageous further developments of the invention can be derived from the remaining dependent claims and from the following description of the embodiments with reference to the figures.